Lighting devices for motor vehicles usually have LED or laser diodes and current drivers. Various implementation options exist for lighting devices of this kind. On the one hand, a discrete structure consisting of diodes and driver blocks in one or more sub-modules can be selected and secondly, fully integrated circuits can also be selected. In addition, mixed solutions with discrete and fully integrated circuit elements are also possible.
Lighting facilities, in particular those used in the automotive field, are subject to specific constraints. Thus, they are usually expected to be both inexpensive and compact. In addition though, they must also be robust and should cause as little electromagnetic interference as possible.
Drivers of lighting devices in the automotive field must be able to be operated in on-board networks of motor vehicles. These typically vary between 9 and 16 V. Depending on the particular active consumers, the battery or on-board power supply voltage can collapse to a varying extent. Particularly salient in this regard is the on-board power supply voltage, for example, when operating the ignition or starter. At the output of the driver, either a constant current or a current which is controlled as a function of input variables must be delivered at all times.
Examples of suitable drivers are switched DC/DC converters. These are usually relatively complex and include a housing and appropriate cabling, which can be relatively space-consuming. Due to the switched-mode operation, EMC protection is additionally required, which also increases the costs of the driver. Their advantage, however, is that the electrical losses are usually small. These are equal to around 10% of the output power, for example. When operating two light-emitting diodes, for example, which each have a forward voltage of 3.5 V and are supplied with a current of 1 A, the power loss is given by PV=×3.5V×1 A×10%=0.7 W. Due to their low loss, the switched converters are used in particular for light sources with high luminous flux, e.g. dipped-beam and main beam lights.
Alternatively, linear regulators or so-called resistance regulators (series resistor in series with an LED across a voltage source) are now widely used as a cost-effective solution with lower requirements. They cause relatively low levels of electromagnetic interference. At present, however, their scope is limited by their high power losses, which is given by the product of output current and voltage drop. If the two LEDs mentioned above are operated on an on-board network with a supply voltage of 13 V, then the voltage drop across the driver is 13 V−2×3.5 V=6 volts. The power loss is therefore 6 V×1 A=6 W. Nevertheless, it would be desirable to be able to use such resistance and linear regulators, which are smaller, cheaper and cause less interference than switching converters, for lighting equipment, in particular in the automotive sector.
Patent document DE 10 2013 201 766 A1 discloses a lighting device with several semiconductor light sources and a device for operating the semiconductor light sources. The device has switching means, by means of which the semiconductor light sources are divisible into groups for operation with the device. In particular, the sub-division is performed as a function of the on-board supply voltage.